Although the technology for quantum information processing is still at an early stage, numerous applications for such technology have been envisaged and investigated theoretically and experimentally. Quantum computing is one of the most important applications. Applications have also been proposed in the fields of cryptography, communication, and navigation, among others.
A variety of different physical devices have been proposed as host environments for quantum information processing. A common characteristic of these environments is that they can support qubits or similar quantum mechanical systems, and that the qubit (or the like) has a coherence time that is long enough to permit quantum computations to take place.
A qubit is a physical system that has two quantum mechanical states, and that can exist in a superposition of those two states. The possibility of superposition of states is an essential feature of quantum information processing. The two states of a qubit are often represented in Dirac notation by the symbols |0> and |1>, respectively.
Another important feature in many aspects of quantum information processing is entanglement. Two particles are said to be entangled if the quantum state of one cannot be described without reference to the other. Stated more formally, a system is entangled if its quantum state cannot be factored as a product of the individual states of its constituent particles. As a consequence of entanglement, the outcome of an experiment that collapses the quantum state of a first particle to produce an observable measurement can be correlated with the outcome of a similar experiment performed on a second particle that is entangled with the first, even if at the time of measurement the particles are separated by a macroscopic distance that precludes mutual interaction.
Researchers have considered numerous quantum mechanical systems in their quest for two-state systems that would be promising as qubits for quantum information processing. Among the more promising systems are neutral atoms that have conveniently spaced hyperfine levels. The hyperfine structure of an atom is the splitting of the energy of an electronic orbital into multiple levels due to electrical and magnetic interactions between the electron and the atomic nucleus. In some atoms, there are hyperfine splittings that provide pairs of energy levels suitable for use as a qubit.
The hyperfine splitting can also be utilized to prepare ensembles of entangled atoms. The ensembles may consist of pairs of atoms, or of groups of three or even more atoms. In an idealized example, each of a pair of atoms are initially prepared in the individual state |1>, so that the joint state is the separable (hence not entangled) state|11>=|1>⊗|1>.   (1)
An appropriate excitation then excites one (unidentifiable) member of the pair, to produce the entangled state(1/√2)·(|01>+|10>).   (2)
Further excitation can produce, e.g., the entangled state(1/√2)·(|00>+|11>).   (3)
A phenomenon referred to as Rydberg blockade has been utilized to prepare pairs of atoms in entangled states such as the state described by Equation (2). Atoms that are excited to very high principal quantum numbers n are referred to as Rydberg atoms. Rydberg atoms exhibit a strong mutual electric dipole-dipole interaction (EDDI). One consequence of the EDDI is that when an appropriately tuned optical pulse excites a first atom to a Rydberg state, the EDDI can detune a second atom situated within a dipole interaction distance from the excitatory pulse, so that the second atom remains behind in the initial state. Subsequent evolution of the two-atom system over one pathway for the Rydberg atom and a different pathway for the non-Rydberg atom (of course the two atoms must be indistinguishable) can lead to the entangled state.
Although the Rydberg blockade method has produced interesting results, alternatives have been sought, not least because it is difficult to maintain the required phase coherence between the highly excited Rydberg atoms and the atoms in the initial state.
One proposed alternative is to utilize so-called Rydberg-dressed interactions in place of the Rydberg blockaded interactions described above. In a Rydberg-dressed interaction, a small amount of Rydberg character is admixed into the atomic ground state to produce a Rydberg-dressed atom. The Rydberg-dressed atom still exhibits EDDI which has the desired effect of producing a two-atom system that can evolve into an entangled state. However, phase control is more robust because phase coherence now only need be maintained between ground-state atomic levels, which are far less sensitive to thermal fluctuations that affect the optical phase.
Although there are theoretical benefits to the use of Rydberg-dressed interactions, experimental demonstration has been elusive. Hence there remains a need for a new, practical approach that can reliably produce entangled pairs of neutral atoms using Rydberg-dressed blockade.